PROJECT SUMMARY/ABSTRACT Autism spectrum disorder (ASD) and fragile X syndrome (FXS) are two disorders associated with pervasive and lifelong deficits. ASD occurs in approximately one out of every 59 children. FXS is a genetic disorder and is the most common inherited cause of intellectual disability. In addition, FXS is the most common genetic cause of ASD. Boys with FXS and many boys with ASD demonstrate deficits in grammar and vocabulary skills in addition to social communication deficits. Both ASD and FXS are also associated with increases in anxiety disorders and atypical physiological arousal. Deficits in these areas may impact communication skills, particularly when interacting with an unfamiliar communication partner. Familiarity with the communication partner (e.g., child's mother versus an unfamiliar examiner) may impact physiological arousal in these clinical populations. In addition to understanding differences in language performance and physiological arousal across communication partners, it is critical to consider factors such as ASD severity and anxiety in these clinical populations, including the impact of these variables on language. It is unclear whether anxiety symptoms, physiological arousal, and ASD symptoms relate to language performance depending on the communication partner. This project will address two specific aims: 1) to examine how communication partner impacts language performance and physiological arousal in boys with ASD and boys with FXS, and 2) to investigate the relationship between physiological arousal, anxiety symptoms, ASD severity and language performance in boys with ASD and boys with FXS during interactions with two different communication partners. This project will include 20 boys with ASD and 20 boys with FXS, all between nine and 18 years of age. To assess language performance, the participants will complete two conversational language samples, one with their mother and one with an unfamiliar examiner. Participants will wear wristbands tracking physiological arousal during the conversations. In addition, all participants will complete an assessment to quantify ASD symptom severity and parents will complete a parent report form to measure anxiety symptoms. From a public health perspective, this work will improve assessment methods for boys with ASD and boys with FXS by providing important insight regarding how to interpret speech and language assessments utilizing conversational language samples with different communication partners. This project will provide important theoretical contributions to our knowledge regarding biobehavioral models; specifically, this work will shed light on the relationship between language performance, physiological arousal, ASD symptoms and anxiety in boys with ASD and boys with FXS.